OBJECTIVES: This study aimed to evaluate the diagnostic accuracy of Cyclin D1 as an adjunct immunomarker in CD99 positive small round cell neoplasms with primary consideration of PNET/EWS.

MATERIALS AND METHODS: Tissue from 2017 to 2023 with a histopathologic diagnosis of CD99 positive small round blue cell tumors with primary consideration of Primitive Neuroectodermal Tumor (PNET)/Ewing Sarcoma were retrieved and Cyclin D1 immunohistochemical staining done. Diagnostic accuracy of Cyclin D1 immunostaining was determined by calculating the sensitivity, specificity, positive predictive value, and negative predictive value.

RESULTS: Cyclin D1 immunohistochemical staining was performed in 19 specimens available, of which 13 yielded a positive result. Of these, 8 had a final histopathologic diagnosis of CD99 positive small round blue cell tumor with primary consideration of PNET/Ewing Sarcoma, resulting in sensitivity of 61.54%, specificity of 100%, positive predictive value of 100% and negative predictive value of 50.0%. The overall accuracy is 72.2%.

CONCLUSION: Cyclin D1 can be used as an adjunct immunomarker to aid in the diagnosis of CD99 positive round cell tumor with primary consideration of PNET/Ewing Sarcoma specifically in resource limited settings where molecular testing is not readily available. Given the high specificity of Cyclin D1 in such cases, it can be used to rule out other small round blue cell tumors that can also stain positive for CD99 such as Rhabdomyosarcoma. However, interpretation must be done in conjunction with the results of other immunohistochemical stains in order to increase its diagnostic accuracy.